intel/eu: Split brw_inst ex_desc accessors for SEND(C) vs. SENDS(C).

intel/eu: Split brw_inst ex_desc accessors for SEND(C) vs. SENDS(C).

The brw_inst opcode accessors are going away in one of the following
commits.  We could potentially replace them with the new helpers that
do opcode remapping, but that would lead to a circular dependency
between brw_inst.h and brw_eu.h.  This way we also avoid ordering
issues that can cause the semantics of the ex_desc accessors to change
depending on whether the ex_desc field is set after or before the
opcode instruction field.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>